The present invention relates to a fuel injection system for an internal combustion engine which corrects fuel amount in accordance with temperature of intake air in or near engine cylinders.
It is known that a fuel injection control system is equipped with a pressure sensor for detecting an intake pressure PM in an intake pipe downstream of a throttle valve and a revolution sensor for detecting a rotational speed Ne or r.p.m. of an engine so that a basic fuel injection amount Tp may be computed according to the detected signals from the two sensors. The basic amount Tp is then corrected with an intake air temperature or the like to suppress the discrepancy of the air/fuel ratio thereby to improve the purification of engine exhaust gases. If the engine r.p.m. is constant, for example, the fuel to be fed is increased with the rise in pressure PM in the intake pipe during acceleration. On the other hand, the temperature of the air sucked into the intake pipe from an air cleaner is measured as the intake temperature for use in correcting the basic fuel amount, and the sensor for this measurement is usually disposed upstream of the throttle valve as is freed from the influences of the combustion temperture in the engine cylinder. This system cannot accurately correct the fuel amount with the intake temperature because the intake air is heated through aheat transfer from the cylinder wall of the internal combustion engine and the density of intake air is varied.
As a matter of fact, more specifically, the place which is the most liable to be thermally influenced by the running state of the engine is the cylinder, and the air having passed through the intake pipe is sucked into the cylinder through an intake valve disposed in the head of the cylinder. As a result, the temperature of air actually sucked into the cylinder will highly vary with the temperature in the cylinder.
For example, when the throttle valve is abruptly opened into an acceleration state while holding a constant r.p.m. a great amount of air is promptly sucked into the intake pipe so that the pressure in the intake air will promptly rise stepwise as shown in (a) of FIG. 13. As has been described hereinbefore, the pressure PM in the intake pipe is detected by the pressure sensor so that the basic fuel injection amount Tp is determined on the basis of the pressure value PM at this time and the engine revolution number Ne. At the same time, the fuel is increased in conformity to the pressure variations in the intake pipe while compensating the fuel wetting the intake port so that a constant air/fuel ratio may be maintained at all times.
In the acceleration state of (a) of FIG. 13, however, the temperature in the intake pipe has its temperature rather dropped by the flow of the increased intake air, as seen from (b) of FIG. 13, but the air in the vicinity of the cylinder is gradually heated to a high temperature by the intensive combustion in the cylinder. In case, on the other hand, the high load run at this time is abruptly released, the flow velocity in the intake pipe restores its initial value, and the temperature in the intake pipe rises to its initial level. On the other hand, the rising rate of the air temperature in the cylinder during the acceleration is far slower than that of the pressure in the intake pipe, which instantly responds to the motion of the throttle valve. The falling rate of the air temperature in the cylinder during deceleration is also far slower than that of the pressure in the intake pipe. This is because it takes a considerable time for the cylinder itself to be heated or cooled in accordance with the intensity of the combustion state in the cylinder. As has been described above, the variations in the air temperature at the cylinder shown in (b) of FIG. 13 are slower than those in the pressure in the intake pipe shown in (a) of FIG. 13.
A stoichiometric air/fuel ratio is obtained, as shown in (c) of FIG. 13, while a high load run continues so that the air temperature in the cylinder is stable. When the air temperature in the cylinder is in its rising course, the air temperature in the cylinder is still low to give a high air density, that is, an excessive air to the fuel which is set in accordance with the pressure in the intake pipe so that the air/fuel ratio is shifted for several tens seconds to the lean side. At the end of the deceleration, on the contrary, the air temperature in the cylinder is still high, although the throttle valve is returned to allow the pressure in the intake pipe to restore its initial value so that the fuel flow rate is dropped. As a result, the air density is still low to give a smaller air flow rate to the fuel so that the air/fuel ratio is shifted for several tens seconds to the rich side. These shifts of the air/fuel ratio will invite deterioration in the exhaust gas emission.
There is also known a technique of compensating the fuel amount while inferring the temperature of intake air flowing in the intake pipe, as is disclosed in Japanese Patent Laid-Open No. 60-90933. In this fuel rate correction by the intake air temperature of Japanese Patent Laid-Open No. 60-90933, too, the injection rate of the fuel is determined while leaving it impossible to compensate for the density variations due to the temperature variations of air.
In order to eliminate this problem, moreover, it is conceivable to measure the air temperature in the cylinder directly by attaching a temperature sensor to the cylinder wall. It is remarkably difficult to realize a temperature sensor which sensitively responds to the various running conditions of the engine while retaining sufficient durability.
It is therefore an object of the present invention to provide a fuel injection system which accurately control the air/fuel ratio no matter what the running state is, by considering the intake air temperature in or near the engine cylinder.